Donnelly (A Northern Light) melds contemporary teen drama with well-researched historical fiction and a dollop of time travel for a hefty read that mostly succeeds. Andi Alpers is popping antidepressants and flunking out of her Brooklyn prep school, grieving over her younger brother's death. She finds solace only when playing guitar. When the school notifies her mostly absent scientist father that she's flirting with expulsion, he takes Andi to Paris for Christmas break, where he's testing DNA to see if a preserved heart really belonged to the doomed son of Louis XVI and Marie Antoinette. Andi is ordered to work on her senior thesis about a (fictional) French composer. Bunking at the home of a renowned historian, Andi finds a diary that relates the last days of Alexandrine, companion to (you guessed it) the doomed prince. The story then alternates between Andi's suicidal urges and Alexandrine's efforts to save the prince. Donnelly's story goes on too long, but packs in worthy stuff. Musicians, especially, will appreciate the thread about the debt rock owes to the classics. Ages 14–up. (Oct.)